1. Field of the Invention
This invention relates to digital wireless communication systems, and more particularly to methods for detecting forward and reverse link imbalances in digital wireless communications systems.
2. Description of Related Art
Wireless communication systems facilitate two-way communication between a plurality of subscriber mobile radio stations or xe2x80x9cwireless unitsxe2x80x9d and a fixed network infrastructure. Typically, the wireless units communicate with the fixed network infrastructure via a plurality of fixed base stations. Exemplary systems include such mobile cellular telephone systems as Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA) systems, and Frequency Division Multiple Access (FDMA) systems. The objective of these digital wireless communication systems is to provide communication channels on demand between the wireless units and the base stations in order to connect the wireless unit users with the fixed network infrastructure (usually a wired-line system).
Wireless units typically communicate with base stations using a duplexing scheme that allows for the exchange of information in both directions of connection. Transmissions from a base station to a wireless unit are commonly referred to as xe2x80x9cdownlinkxe2x80x9d transmissions. Transmissions from a wireless unit to a base station are commonly referred to as xe2x80x9cuplinkxe2x80x9d transmissions. In CDMA and FDMA communication systems, the downlink is commonly referred to as the xe2x80x9cforwardxe2x80x9d link and the uplink is commonly referred to as the xe2x80x9creversexe2x80x9d link. A well-known problem in cellular communication systems is system performance degradation caused by signal strength imbalances in the forward and reverse links. To mitigate this problem, cellular communication system designers attempt to ensure that signal path losses tolerated by the reverse links are equal to or approximately equal to those tolerated by the forward links. One important design objective is to balance the forward and reverse links. Unfortunately, due to dynamically changing network conditions such as system loading, antenna pattern mismatches, differences in antenna gains, and other channel variations, imbalances still occur. In cellular communication systems such as CDMA and FDMA, forward and reverse link imbalances often cause degraded system performance.
Therefore, balancing the forward and reverse links is a very important design goal in wireless digital communication systems. Unless the links are balanced, system performance is degraded. For example, under weak reverse link conditions (i.e., the reverse link is weaker than the forward link) wireless units attempt to access their associated base stations by generating multiple access probes until all of the access probes are exhausted. These multiple access attempts result in increased channel interference on the reverse link. Under weak forward link conditions (i.e., the forward link is weaker than the reverse link) wireless units are unable to receive acknowledgment messages on their associated forward links. Consequently, the wireless units will not declare service, initiate calls, nor respond to base station orders.
Unfortunately, link imbalances are indiscernible by the prior art wireless units. Consequently, the prior art wireless units exhibit undesirable behavior in the presence of link imbalances. For example, in a weak reverse link condition, the prior art wireless units can become locked into a digital mode of operation when the digital system is, in fact, unavailable for service. This occurs when the wireless unit receives a strong signal on the forward link paging channel. However, the reverse link is weak.
Although the wireless unit is unable to register or originate calls on the reverse link, it believes that digital service is available due to the strong paging channel signal. Therefore, even though an alternative analog system may be available, the mobile is locked into a useless digital mode of operation. Performance also degrades when the reverse link is stronger than the forward link. Under these conditions, the wireless unit can communicate with the base station. However, because of the relatively weak forward link, the wireless unit cannot decipher the control information transmitted by the base station. In either scenario, calls are disadvantageously lost, and system call delivery rates are reduced. A better understanding of the performance problems created by link imbalances can be obtained by briefly reviewing simple call flow examples in a CDMA communication system.
CDMA Call Flow Examples and CDMA Call Handshake Protocols
Tables 1 and 2 show simple call flow examples as set forth in the Telecommunications Industry Association (TIA) specification governing the operation of CDMA wireless unit and base station equipment. The TIA specification is entitled xe2x80x9cWireless unit-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,xe2x80x9d TIA/EIA/IS-95-A, was published in May 1995 by the Telecommunications Industry Association, and is referred to hereafter as the IS-95 specification. As set forth in the IS-95 specification, Tables 1 and 2 follow the following conventions:
All messages are received without error.
Receipt of messages is not shown (except in the handoff examples).
Acknowledgements are not shown.
Optional authentication procedures are not shown.
Optional private long code transitions are not shown.
Table 1 shows a simple call flow example wherein a wireless unit originates a call. Messages are transmitted from the wireless unit to the base station using the access channel. Messages are transmitted from the base station to the wireless unit using the paging channel. As shown in Table 1, the wireless unit first detects a user-initiated call, and then sends an xe2x80x9coriginationxe2x80x9d message via the CDMA access channel. The access channel is a slotted random access channel. The wireless unit transmits on the access channel using a random access procedure. Many parameters of the random access procedure are supplied by the base station in an access parameters message. The entire process of transmitting one message and receiving (or failing to receive) an acknowledgement for that message is called an xe2x80x9caccess attempt.xe2x80x9d Each transmission in the access attempt is called an xe2x80x9caccess probe.xe2x80x9d Within an access attempt, access probes are grouped into access probe sequences. Each access probe sequence comprises a fixed number of access probes. The first access probe of each access probe sequence is transmitted at a specified power level relative to the nominal open loop power level. Each subsequent access probe is transmitted at a power level that is a specified amount higher than the previous access probe.
During normal CDMA operation, when a wireless unit user initiates a phone call, the wireless unit sends an access probe to the base station. If the base station properly receives the access probe, the wireless unit should receive back an acknowledgement from the base station. Once the wireless unit receives the acknowledgement, the wireless unit is instructed by the base station to wait and to stop sending further access probes to the base station. This is necessary because too many access probes will produce undesirable interference on the communication channel. The wireless unit therefore waits until it is assigned a communication channel by the base station. As shown in Table 1, the base station informs the wireless unit of the channel assignment by sending a channel assignment message via the paging channel.
Once the wireless unit receives its channel assignment from the base station, it changes its receive and transmit frequencies to the assigned channel. The wireless unit then attempts to initiate communication on the assigned channel by establishing or xe2x80x9csetting upxe2x80x9d the traffic channel. If the traffic channel initialization is successful, the wireless unit then acquires the traffic channel. The wireless unit then begins sending a traffic channel preamble. As shown in Table 1, the base station acquires the reverse traffic channel and sends a base station acknowledgement order to the wireless unit if the reverse traffic channel was properly acquired. At this point, the wireless unit and the base station begin negotiating service. The communication link can fail at any point during this negotiation process. However, if the negotiation process is successful, communication commences, and a telephone conversation commences.
Table 2 shows a simple call flow example wherein a wireless unit terminates a call. As shown in Table 2, during normal operation, when a call is initiated by a base station the base station sends a page or slotted page message to the wireless unit via the paging channel. The wireless unit then sends a page response message to the base station via the access channel. The base station then establishes a traffic channel and begins sending null traffic channel data to the wireless unit. The base station then sends a channel assignment message to the wireless unit via the paging channel. As described above with reference to Table 1, once the wireless unit receives its channel assignment from the base station, it changes its receive and transmit frequencies to the assigned channel. The wireless unit then attempts to initiate communication on the assigned channel by setting up the traffic channel. As before, if the traffic channel initialization is successful, the wireless unit acquires the traffic channel and processes primary traffic. Soon thereafter, if the communication negotiation is successful, communication commences via the forward and reverse channel pair. With the call flow examples in mind, it is now possible to more fully describe the problems created by link imbalances in a cellular communication system.
Call Delivery Failures due to Link Imbalances
Under some network conditions, the forward link is intentionally made stronger than the reverse link resulting in an extended CDMA forward link coverage area. In areas with neighboring analog cells, it is possible for a dual mode wireless unit to receive a valid signal on the CDMA paging channel while in the analog coverage area. As described above, this causes the wireless unit to lock into a digital (in this case, CDMA) mode of operation. Disadvantageously however, the wireless unit will be unable to register or originate a call within the CDMA cell because it is beyond its reverse link coverage range. Stated another way, under these network conditions, wireless units become confused into relying upon valid CDMA coverage when, in fact, there is none. Disadvantageously, the wireless unit is unaware of the link imbalance problem. Rather than locking into a digital operational mode, the dual mode wireless units preferably should remain in an analog operational mode.
In contrast, network conditions exist wherein the reverse link is stronger than the forward link. For example, in personal communication systems (PCS), the reverse link is favored due to weak coding characteristics of the link and also due to inherent limitations of the high power amplifier (HPA). The coding characteristics of the 13 kb/s link can cause the forward link to be weaker than the reverse link. Coding for the 13 kb/s PCS systems is not as robust or efficient as is the coding for the 8 kb/s systems. In addition, the HPA has a limited amount of power, and therefore can cause the forward link to be weaker than the reverse link under certain circumstances. Consequently, calls fail either during call setup due to the fading characteristics of the paging channel or during forward traffic channel initialization.
Other factors contribute to link imbalance conditions. Link imbalances can be caused by variations in base station antenna gains and antenna pattern mismatches. In addition, greater path loss in the forward link relative to the reverse link can cause the forward link to be weaker than the reverse link. Additionally, co-channel interference from neighboring base stations can result in weaker forward links. In accordance with the IS-95 specification, each CDMA base station continuously transmits an unmodulated, direct-sequence spread spectrum signal referred to as the xe2x80x9cpilot channelxe2x80x9d. The pilot channel is transmitted at all times by the base station on each active forward CDMA channel. In addition to facilitating other wireless unit functions, the pilot channels allow the wireless units to perform signal strength comparisons between base stations. Unfortunately, the pilot channels of nearby base stations can interfere with one another resulting in weak forward channel strengths. The interfering pilot channels may or may not be on a wireless unit""s neighbor list.
Also, weak forward link conditions may be caused by interference sources that are external to the CDMA system. Finally, weak forward links can occur due to inadequate traffic channel power allocation at initialization. Weak forward channel conditions are characterized by poor paging channel performance, which can cause the paging channels to be lost while the wireless unit is in the System Access State. Weak forward channel conditions can also lead to Traffic Channel Initialization failures, failures in receiving Channel Assignment messages, and failures in receiving Base Station Acknowledgement Orders.
Whether a wireless unit is attempting to originate or to terminate a call, the call can be lost due to link imbalances. In both cases, call delivery rates will suffer. In addition, system performance is adversely affected when active in-progress calls (i.e., wireless units having active traffic channels carrying in-progress calls) are dropped due to link imbalance conditions. Therefore the need exists for a method and apparatus that detects link imbalances and instructs the wireless unit to process calls in accordance with the detection.
The need exists for a method and apparatus that can detect link imbalances in a cellular communication system and process calls accordingly. The need exists for a means for determining whether in-progress calls are dropped due to link imbalance conditions, and if so, for taking appropriate corrective action. The present invention provides such a method and apparatus. The present invention detects link imbalances in a cellular communication system, determines the relative strengths of the forward and reverse links, and processes calls (originated, terminated and dropped calls) according to the determination.
The present invention is a novel method and apparatus for detecting forward and reverse link imbalances in a digital cellular communication system and for processing calls accordingly. The present invention improves call delivery rates in a digital cellular communication system by detecting forward and reverse link imbalances, determining which link is weaker, and processing calls in accordance with the determination. The present invention improves system performance by determining whether in-progress calls are dropped due to link imbalance conditions. If so, the present invention takes corrective action during subsequent system accesses.
In a weak reverse link condition, the wireless unit receives a strong received signal strength indication (RSSI) on the forward link, but is unable to communicate using the access channel. Each time the wireless unit attempts to access the digital system, it exhausts all of its assigned access probes, and thereby fails the access process. In contrast, weak forward link conditions are characterized by poor paging channel performance, resulting in the loss of the paging channel. The wireless unit is unable to initiate or receive calls because it cannot receive control information via the paging channel.
The present inventive method determines whether a failure has occurred in either the forward or reverse link. If a failure occurs in either link, the present inventive method determines whether the failure is due to a link imbalance condition. When link imbalances are detected, the present invention uses the link failure cause to determine which link is weaker. Based on this determination, the present invention takes corrective action to establish or re-establish calls. More specifically, in one embodiment of the present invention, the method detects weak reverse link conditions by determining whether a wireless unit has attempted to access a selected base station a predetermined number of times (i.e., whether a maximum number of access probes has been transmitted by the mobile). If so, the method of the present invention switches operation to a secondary digital system (or to an analog system if the secondary digital system attempt fails). In one embodiment, the present inventive method detects weak forward link conditions by detecting failures in either the paging channel or the traffic channel.
If the paging channel is lost, the method determines whether a traffic channel initialization (TCI) timeout occurred. If not, the mobile returns to normal operation, and no link imbalance is found. However, if a TCI timeout occurred, the wireless unit determines what caused the paging channel to be lost. The present method tests the pilot strength to determine whether the paging channel was lost because the wireless unit is too far from the base station, or because of significant interference on the paging channel. The method tests to determine whether another strong pilot channel exists. If so, the method instructs the wireless unit to perform an idle handoff and establish communication with the strong pilot channel. If not, the method causes the wireless unit to perform a xe2x80x9cnew system exit.xe2x80x9d
The details of the preferred and alternative embodiments of the present invention are set forth in the accompanying drawings and the description below. Once the details of the invention are known, numerous additional innovations and changes will become obvious to one skilled in the art.